In mobile communication systems, the wireless fading channels are time-varying. In order to effectively detect signals, channel information is needed to implement relevant detection. In order to obtain the channel information, the base station transmits the signals or sequences, which are known to the UE, via wireless channels. And the UE obtains the channel information via detecting the known signals or sequences. The known signals or sequences are called pilot signals or reference signals.
A Long Term Evolution/Long Term Evolution-Advanced (LTE/LTE-A) system is a typical correlation detection system. There are seven kinds of pilot signals or reference signals according to the purposes as described below.
A Cell-specific Reference Signal (CRS) is used for data demodulation of PBCH, PDCCH, and some of PDSCHs, measurement of Channel State Information (CSI) of downlink shared channel (PDSCH) of transmission modes 1 to 8.
A Multimedia Broadcast Multicast Service Single Frequency Network Reference Signal (MBSFN-RS) is used for data demodulation of a Multicast Channel (MCH).
A downlink User Equipment-specific Reference Signal (DMRS) is used for data demodulation of PDSCH of transmission modes 7 to 10.
A Channel State Information Reference Signal (CSI-RS) is used for measurement of the CSI for PDSCH of transmission modes 9 to 10.
A Positioning Reference Signal (PRS) is used for performing positioning function.
An uplink Demodulation Reference Signal (DMRS) is used for data demodulation of physical uplink shared channel (PUSCH) and physical uplink control channel (PUCCH).
An uplink Sounding Reference Signal (SRS) is used for measurement of uplink CSI and obtaining downlink channel information via channel reciprocity for TDD systems.
Besides CSI measurement, data demodulation and synchronization assistance, the CRS is further used for measurement of Reference Signal Receive Power (RSRP/RSRQ) to implement path loss estimation, cell association, etc.
Reference signal received power (RSRP/RSRQ), is defined as the linear average over the power contributions (in [W]) of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth.
For RSRP/RSRQ determination the cell-specific reference signals R0 according to TS 36.211 shall be used. If the UE can reliably detect that R1 is available it may use R1 in addition to R0 to determine RSRP/RSRQ, where R0 and R1 are reference signal of CRS port 0 and port 1, respectively.
Reference Signal Received Quality (RSRQ) is defined as the ratio K×RSRP/RSRQ/(E-UTRA carrier RSSI), where K is the number of RB's of the E-UTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks.
E-UTRA Carrier Received Signal Strength Indicator (RSSI), comprises the linear average of the total received power (in [W]) observed only in OFDM symbols containing reference symbols for antenna port 0, in the measurement bandwidth, over K number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc. If higher-layer signalling indicates certain subframes for performing RSRQ measurements, then RSSI is measured over all OFDM symbols in the indicated subframes.
An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. There is not necessarily a one-to-one relationship between the antenna port and physical antenna. The signal of each antenna port may be transmitted by combining signals over one antenna or multiple antennas. This process is called port virtualization.
With development of antenna technologies, particularly active antenna array technologies, physical antenna elements are not only distributed in a horizontal dimension, but also distributed in both the vertical dimension and the horizontal dimension to form a 2-Dimension (2-D) antenna array. As shown in FIG. 1 and FIG. 2, FIG. 1 is a schematic diagram illustrating structure of an active antenna cross-polarized 2-D antenna array in accordance with an embodiment of the prior art. FIG. 2 is a schematic diagram illustrating structure of a co-polarized 2-D antenna array in accordance with an embodiment of the prior art. N is the number of antennas in a horizontal co-polarized direction and M is the number of antennas in the vertical dimension.
Using the 2-D antenna array, the vertical beamforming and/or dynamic vertical sectorization is possible. Narrow beams, which directly point to the UE or avoid interfere to the UE using the same time frequency resource, are generated in the vertical dimension via multiplexing different weighting factor to each antenna. Therefore, the received signal power may be improved or the interference to the other UEs using the same time frequency resources may be avoided.
At present, although the LTE/LTE-A system supports that different reference signals may adopt different port virtualization methods, in the LTE/LTE-A specification, only passive antenna array is only taken into consideration in standardization process, the characteristics of the active antenna array are not taken into consideration.
For the convenience of description, the wireless communication system with the active antenna array is called an active antenna system hereinafter. In order to obtain a vertical beamforming gain, a UE-specific vertical beamforming technology may be adopted for PDSCH. In order to ensure that UE in anywhere of a cell can receive the CRS, a cell-specific vertical beamforming vector may be adopted in the vertical dimension. In this way, the transmission of the CRS and transmission of data, which is measured based on the CSI-RS or demodulated based on the DMRSDMRS, may have different beamforming gains in the vertical dimension, resulting in different received signal strength. FIG. 3 is a diagram illustrating that the CRS and the PDSCH, which is measured based on the CSI-RS or demodulated based on the DMRS, adopt different vertical beamforming in accordance with an embodiment in the prior art.
In LTE/LTE-A systems, cell association is implemented based on a Reference Signal Receiving Power RSRP/RSRQ value reported measured based on CRS port 0. Since the CRS and PDSCH, which is measured based on the CSI-RS or demodulated based on the DMRS, are transmitted with different vertical beamforming technologies, the cell association may be not suitable for PDSCH, which is measured based on the CSI-RS or demodulated based on the DMRS.
FIG. 4 is a diagram illustrating the transmission of the CRS in accordance with an embodiment in the prior art. As shown in FIG. 4, the CRS is virtualized with a fixed vertical beamforming vector. Generally, in order to control interference to the adjacent cells, a down-tilt angle may be to be formed. For instance, in the typical Urban Marco (UMa) scenario, the base station (BS) antenna height is 25 m, and the inter-site distance is 500 m. The UEs are distributed outdoor or in low buildings with height of 18˜24 m. The 90 degree represents the horizontal direction, the 0 degree represents vertically pointing to the sky, and the 180 degree presents vertically pointing to the ground. Then, electronic down-tilt of CRS is 102 degree. In that case, the CRS vertical antenna array gain received by the UE is much small if the UE antenna height is larger than or equal to the BS antenna height. The signal strength of the CRS received by the UE is relatively weak no matter of accessing which cell. The CRS receiving for these UEs are in deep spatial fading.
On the other hand, the RSRP/RSRQ value measured based on the CRS port 0 is used to estimate the path loss value for the uplink power control. Inappropriate RSRP/RSRQ report may affect performance of the uplink control in the active antenna systems. In LTE/LTE-A standardization with passive antennas, it is assumed that the uplink path loss and downlink path loss is approximately symmetrical. This is because in the passive antenna system, the uplink channel and downlink channel may experience the same large-scale fading and similar antenna array gain. However, in the active antenna systems adopting the vertical beamforming, if the CRS adopts the fixed vertical beamforming, while the Maximal Ratio Combining (MRC) detection is used in the uplink receiving, the antenna gains for the uplink and downlink are asymmetric. Therefore, the downlink path loss estimated with the RSRP/RSRQ value is significantly different from the actual uplink path loss and the uplink power control performance may be further affected.